Transient voltage suppressors (TVS) are devices used to protect integrated circuits from damage caused by over voltage imposed onto the integrated circuit. An integrated circuit is designed to operate over a normal range of voltages. However, in situations such as electrostatic discharge (ESD), electrical fast transients and lightning, an unexpected and an uncontrollable high voltage may accidentally strike onto the circuit. The TVS devices are required to provide protection to circumvent the damages that are likely to occur to the integrated circuits when such over voltage conditions occur. As an increasing number of devices are implemented with integrated circuits that are vulnerable to over voltage damages, demands for TVS protection are also increased. Exemplary applications of TVS can be found in USB power and data line protection, digital video interfaces, high speed Ethernet, Notebook computers, monitors and flat panel displays.
Unidirectional TVS devices are widely used for protecting the integrated circuits of applications described above. Such devices are defined by their operation. During a transient's positive cycle (i.e., positive voltage spike) a unidirectional TVS device is reverse biased. The device operates in avalanche mode and directs the transient current to the ground. The transient is also clamped at the clamping level provided by the TVS device, thereby ensuring protection to the attached integrated circuit. During a transient's negative cycle (i.e., negative voltage spike) the unidirectional TVS device is forward biased. The transient is clamped at the built-in voltage drop of the unilateral device, and current is conducted in the forward direction.
Conventional unidirectional TVS devices employ an NPN transistor with the base and emitter shorted to provide the functionality of the unidirectional device. These are typically used for applications requiring a clamping voltage below 5 V. However, in order to achieve effective protection for applications below 3.3 V (e.g., 3.3 V, 2.4V, or 1.8V), the base (i.e., p-layer) of the NPN transistor must be very lightly doped. Because the clamping voltage of the unidirectional TVS device is so heavily dependent on the doping concentration of the base layer, any slight variations in the processing/manufacturing steps may greatly affect the performance/behavioral characteristics of the unidirectional device. As such, there is need in the art for a unidirectional TVS device that supports applications that operate below 5 V.
It is within this context that embodiments of the present invention arise.